Various types of coil apparatuses have been conventionally proposed and come into practical use. As one of such apparatuses, a coil apparatus which can be applied as an in-vehicle antenna or a transponder has been recently proposed. In the coil apparatus which is applied to such an intended purpose, a reduction in size is demanded, and a stable inductance in a working frequency range desired by a customer is also demanded. Therefore, there has been also devised a divided winding conformation in which a coil portion obtained by forming layers of a winding in a radial direction is divided in a direction of an axial center of a core so that divided coil portions are formed.
That is, in the divided winding conformation described in Patent Reference 1 mentioned above, although a flange integrally formed with the core is provided between adjacent coil portions, a further reduction in size and a decrease in core manufacturing cost can be achieved if such a flange can be eliminated, which will be more preferable.
However, when the divided winding conformation is accepted without providing the flange and a plurality of coil portions are sequentially formed, a winding of a precedently formed coil portion may possibly collapse during formation of a next coil portion.
Furthermore, in a coil apparatus which is applied to an intended purpose such as an in-vehicle antenna or a transponder, a ferrite core having excellent high-frequency characteristics is generally used. Moreover, a coil is wound around this ferrite core for the required number of times, a coil end is connected with a metal terminal provided at each of both ends of the ferrite core in a longitudinal direction, and an entire structure is covered with a thermosetting resin such as an epoxy resin.
As the ferrite core, it is general to use an elongated one having a large length seen from a direction of a winding axis of the coil so that an inductance value, a Q value and self-resonant frequency characteristics and others required in this type of coil apparatus satisfy requested values.
However, the ferrite core is a brittle sintered body, and essentially weak against impact shocks or vibrations. Moreover, the ferrite core must be formed into an elongated shape which is weak against impact shocks and vibrations for the above-described reason. Therefore, in case of an in-vehicle coil apparatus which is constantly exposed to impact shocks and vibrations, how a structure having excellent impact resistant properties and vibration resistant properties is realized is important.
Additionally, in not only the in-vehicle coil apparatus but also a coil apparatus which is used as a communication device inductor or a choke coil, a reduction in size, simplification of structure, a decrease in cost and others are always demanded, and hence how these demands are met is also an important problem.
In this point of view, considering a known technique, for example, Patent Reference 2 discloses a structure in which a synthetic resin base obtained by injection molding is attached at terminal attachment portions provided at both end portions of a ferrite core in a longitudinal direction and a metal electrode terminal is attached at an outer periphery of the synthetic resin base by its own spring action. In this prior art, however, it is difficult to meet demands such as a reduction in size, a simplification of structure, a decrease in cost and others.
As means for solving the above-described problem, Patent Reference 1 discloses a coil apparatus in which ingenuity is exercised with respect to a shape of a ferrite core, a terminal structure or the like to improve frequency characteristics, impact resistant properties and vibration resistant properties.
According to this prior art, a very satisfactory result can be expected in an application in a severe use environment such as an in-vehicle coil apparatus.
Further, as a coil apparatus which is applied to an intended purpose such as an in-vehicle antenna or a transponder, a surface mount type coil apparatus is used, and a reduction in size/thickness, impact resistant properties, vibration resistant properties, heat resistant properties and others are demanded, but an insulating sheath body which covers a core and a coil currently has a cross-sectional shape which is orthogonal to a coil winding direction being formed into a square shape in the surface mount type coil apparatus. Furthermore, in regard to a core accommodated inside, it is often the case that its cross-sectional shape is formed into a square shape in accordance with the insulating sheath body in view of characteristics as a coil.
However, in a coil apparatus in which a core has a square cross-sectional shape, cracks are found in an insulating sheath body in an inspection process in some cases. It can be considered that a coil winding is expanded due to heat when molding the insulating sheath body, a stress is concentrated by expansion at the part of the insulating sheath body which covers square angular portions of the core in particular, and cracks are thereby generated on an outer peripheral surface of the insulating sheath body.
On the other hand, forming the cross-sectional shape of the core into a circular shape with which a stress is hardly concentrated can be considered. However, when a circular cross section which internally touches the original square cross-sectional shape is accepted, a large cross-sectional area of the core cannot be assured, which is not preferable for characteristics. On the other hand, when the cross-sectional shape of the core is selected to be larger than the inscribing circular shape, a preferable wall thickness of the insulating sheath body cannot be assured or the entire coil apparatus is increased in size as a result of putting high priority to assuring a wall thickness because the insulating sheath body has the square cross-sectional shape.
Moreover, in not only the in-vehicle coil apparatus but also a coil apparatus used as a communication device inductor or a choke coil, electrical characteristics are greatly dependent on a core size. In general, better electrical characteristics can be obtained as the core size is increased.
However, since an outside dimension of the coil apparatus is restricted in accordance with its application, when the core size is increased in the restricted outside dimension, a thickness of the insulating covering body formed of a thermosetting resin such as an epoxy resin is relatively reduced, and all or a part of the core or the coil is exposed to the outside, so that impact resistant properties, vibration resistant properties, durability and others as a purpose of insulative covering are thereby hardly guaranteed. On the contrary, when the thickness of the insulating covering body is increased to assure impact resistant properties, vibration resistant properties, durability and others, the core size is reduced this time, thus sacrificing electrical characteristics. That is, in this type of coil apparatus, an important problem is how the core size is increased to assure high electrical characteristics without deteriorating impact resistant properties, vibration resistant properties and durability by insulative covering.
Additionally, considering an influence of the insulating covering body on the core, a structure which does not deteriorate characteristics of the core must be accepted.
In such a point of view, examining a known technique, Patent reference 2 mentioned above discloses a structure in which a synthetic resin base obtained by injection molding is attached at flange portions provided at both end portions of a core in a longitudinal direction and a metal electrode terminal is attached at an outer periphery of the synthetic resin base by its own spring action. However, this prior art does not disclose means for solving the above-described problem.
Although Patent Reference 3 discloses a structure in which an entire structure is covered with a sheath material such as a resin, it does not describe about a resin material constituting the sheath material, and likewise does not disclose means for solving the above-described problem.
Further, examining a known technique, for example, Patent Reference 3 mentioned above discloses a coil apparatus in which an entire structure is covered with a resin mold.
Furthermore, Patent Reference 1 mentioned above discloses a coil apparatus in which an entire structure is covered with an insulating resin and ingenuity is exercised with respect to a shape of a ferrite core, a terminal structure and others to improve impact resistant properties and vibration resistant properties.
Of these prior arts, according to Patent Reference 1 in particular, a very satisfactory result can be expected even in an application in a severe use environment such as an in-vehicle coil apparatus.
Patent Reference 1: Japanese Patent Application Laid-open No. 2003-318030
Patent Reference 2: Japanese Patent Application Laid-open No. 2001-339224
Patent Reference 3: Japanese Patent Application Laid-open No. 130556-1995